Annular blowout preventers are used in drilling wells and are designed to provide a full size opening during operations and when actuated to close on the structure extending therethrough or if no structure is present to close on itself, to prevent a blowout and thus control the well.
In prior blowout preventers, metal inserts have been used in the top surface of the annular packer to prevent extrusion and damage to the packer. Some prior metal inserts have been adapted to have an irising action, i.e., the inserts have a compound motion including a pivotal and a radial motion. Two examples of this type of blowout preventer packer are shown in U.S. Pat. Nos. 3,572,627 and 4,099,699. Other prior structures have used such metal inserts in annular blowout preventer packers and their motion is a simple radial motion. Two examples of this latter type are shown in U.S. Pat. Nos. 3,897,038 and 3,915,424.
One problem which has been encontered with these prior art annular blowout preventers is that when the annular packer deteriorates as a result of exposure to oil base muds the metal inserts can drop into the well bore and if they are hard cast alloy steel they will junk the hole causing drilling to be directed around them by a whipstock or other suitable means.